High-power high-efficiency amplifiers are used to amplify the speech and music frequencies in loudspeaker systems and as high-level modulators for radio transmitters. Such amplifiers are also used as modulated-wave amplifiers in radio transmitters for bandwidths from one Hz to 10 kHz or more at frequencies from less than 100 Hz to several hundred mHz, and in other applications.
Current practice is to use linear class B amplifiers for these applications, with each half of the class B power stage having a continuous linear input-output relationship for a half cycle of one polarity. Such amplifiers are well known to the art and are fully described in "Reference Data for Radio Engineers", 5th edition, New York 1969, pages 17-1 to 17-8 inclusive.
Other types of linear amplifiers have been used, such as the Doherty amplifier described in Terman, "Radio Engineers Handbook", New York 1943, pages 455 to 458 inclusive.
An amplifier using a combination of stepped switched functions was described by C. B. Fisher in U.S. Pat. No. 2,379,513 issued 3 July, 1945.
A linear class B amplifier has an average efficiency of conversion of direct-current power to alternating-current signal power of about 50%. The Doherty and Fisher amplifiers referenced above have somewhat better efficiency, but have practical difficulties that have limited their use in the field.